This invention relates generally to a blue-green light illumination system, and specifically to a ceramic phosphor for converting UV radiation emitted by a light emitting diode (xe2x80x9cLEDxe2x80x9d) to blue-green light.
Semiconductor light emitting diodes are semiconductor chips that are mounted in a package and which emit radiation in response to an applied voltage or current. These LEDs are used in a number of commercial applications such as automotive, display, safety/emergency and directed area lighting.
One important application of semiconductor LEDs is as a light source in a traffic light. Presently, a plurality of blue-green emitting LEDs containing III-V semiconductor layers, such as GaN, etc., are used as the green light of a traffic signal (also known as a traffic light).
Industry regulations often require traffic light colors to have very specific CIE color coordinates. For example, according to the Institute of Transportation Engineers (ITE), a green traffic light in the United States is typically required to have emission CIE color coordinates located within an area of a quadrilateral on a CIE chromaticity diagram, whose corners have the following color coordinates:
a) x=0.000 and y=0.506;
b) x=0.224 and y=0.389;
c) x=0.280 and y=0.450; and
d) x=0.000 and y=0.730.
The following CIE color coordinates are most preferred for green traffic light applications: x=0.1 and y=0.55.
Likewise, industry regulations require automotive display colors to have specific CIE color coordinates. According to the Society of Automotive Engineers (SAE), a green automotive display, such as a vehicle dashboard display, is typically required to have emission CIE color coordinates located within an area of a quadrilateral on a CIE chromaticity diagram, whose corners have the following color coordinates:
e) x=0.0137 and y=0.4831;
f) x=0.2094 and y=0.3953;
g) x=0.2879 and y=0.5196; and
h) x=0.0108 and y=0.7220.
The color coordinates (also known as the chromaticity coordinates) and the CIE chromaticity diagram are explained in detail in several text books, such as on pages 98-107 of K. H. Butler, xe2x80x9cFluorescent Lamp Phosphorsxe2x80x9d (The Pennsylvania State University Press 1980) and on pages 109-110 of G. Blasse et al., xe2x80x9cLuminescent Materialsxe2x80x9d (Springer-Verlag 1994), both incorporated herein by reference.
Presently, GaN based LEDs are designed to emit blue-green light with a peak wavelength of 505 nm, which has the desired CIE color coordinates of x=0.1 and y=0.55. Table I illustrates the optical properties of an LED having an In1-xGaxN active layer that was manufactured according to desired parameters.
In Table I, external quantum efficiency refers to a ratio of a number of photons emitted per number of electrons injected into the LED.
However, these LEDs with the In1-xGaxN active layer suffer from the following disadvantage. Due to frequent deviations from desired parameters (i.e., manufacturing systematic variations), the LED peak emission wavelength deviates from 505 nm, and thus, its CIE color coordinates deviate from the desired x=0.1 and y=0.55 values. For example, the LED color output (e.g., spectral power distribution and peak emission wavelength) varies with the band gap width of the LED active layer. One source of deviation from the desired color coordinates is the variation in the In to Ga ratio during the deposition of the In1-xGaxN active layer, which results in an active layer whose band gap width deviates from the desired value. This ratio is difficult to control precisely during mass production of the LEDs, which leads to inconsistent color coordinates in a given batch of LEDs. Thus, the In1-xGaxN LEDs which are suitable for use in traffic lights have a lower production yield because a large number of such LEDs with unsuitable emission color coordinates have to be discarded. The present invention is directed to overcoming or at least reducing the problem set forth above.
In accordance with one aspect of the present invention, there is provided a blue-green illumination system, comprising a light emitting diode and a luminescent material having emission CIE color coordinates located within an area of a pentagon on a CIE chromaticity diagram, whose corners have the following CIE color coordinates:
e) x=0.0137 and y=0.4831;
b) x=0.2240 and y=0.3890;
c) x=0.2800 and y=0.4500;
g) x=0.2879 and y=0.5196; and
h) x=0.0108 and y=0.7220.
In accordance with another aspect of the present invention, there is provided a traffic light, comprising a housing, at least one lens, a radiation source having a peak emission wavelength of 420 nm and below, and a luminescent material having emission CIE color coordinates located within an area of a quadrilateral on a CIE chromaticity diagram, whose corners have the following CIE color coordinates:
a) x=0.000 and y=0.506;
b) x=0.224 and y=0.389;
c) x=0.280 and y=0.450; and
d) x=0.000 and y=0.730.
In accordance with another aspect of the present invention, there is provided a method of making a blue-green light illumination system, comprising mixing a plurality of starting powders to form a starting powder mixture, firing the starting powder mixture to form a calcined body, converting the calcined body into a phosphor powder having emission CIE color coordinates located within an area of a pentagon on a CIE chromaticity diagram, whose corners have the following color coordinates:
e) x=0.0137 and y=0.4831;
b) x=0.2240 and y=0.3890;
c) x=0.2800 and y=0.4500;
g) x=0.2879 and y=0.5196; and
h) x=0.0108 and y=0.7220.
and placing the phosphor powder into the illumination system adjacent a light emitting diode.